Caesium or cesium[note 1] is a chemical element wi seembolCs an atomic nummer55. It is a saft, sillery-gowd alkali metal wi a meltin pynt o 28°C (82°F), which maks it ane o anly five elemental metals that are liquid at or near ruim temperatur.[note 2] Caesium is an alkali metal an haes pheesical an chemical properties seemilar tae those o rubidium an potassium. The metal is extremely reactive an pyrophoric, reactin wi watter even at −116°C (−177°F). It is the least electronegative element havin a stable isotope, caesium-133. Caesium is mined maistly frae pollucite, while the radioisotopes, especially caesium-137, a fission product, are extractit frae waste produced bi nuclear reactors.
Twa German chemists, Robert Bunsen an Gustav Kirchhoff, discovered caesium in 1860 bi the newly developed method o flame spectroscopy. The first smaa-scale applications for caesium wur as a "getter" in vacuum tubes an in photoelectric cells. In 1967, a specific frequency frae the emission spectrum o caesium-133 wis chosen tae be uised in the definition o the seicont bi the Internaitional Seestem o Units. Syne then, caesium haes been widely uised in atomic clocks.
Syne the 1990s, the lairgest application o the element haes been as caesium formate for drilling fluids. It haes a range o applications in the production o electricity, in electronics, an in chemistry. The radioactive isotope caesium-137 haes a hauf-life o aboot 30 year an is uised in medical applications, industrial gauges, an hydrology. Altho the element is anly mildly toxic, it is a hazardous material as a metal an its radioisotopes present a heich heal risk if released intae the environment.